In a power transmission mechanism configured to transmit rotation of a crank shaft to cam shafts via a chain, the tension of the chain is kept constant by providing a chain tensioner on the slack side of the chain so as to apply an adjustment force of the chain tensioner to the chain.
A general chain tensioner is configured to have a spring and a plunger assembled in a cylinder portion formed in a housing. When the chain slacks, the plunger biased by the spring advances to pressurize the chain, so as to keep an appropriate tension. On the other hand, when the chain produces tension enough to push the plunger, the chain tensioner operates so as to buffer the setback of the plunger, with the aid of hydraulic pressure of a hydraulic oil supplied behind the plunger.
In thus-configured conventional chain tensioner, the chain may produce thereon a large tension depending on the position of the cams when the engine stops, and may thereby largely set back the plunger. In this case, re-start of the engine may sometimes cause an abrupt slack of the chain, thereby the plunger may largely advance to produce an abnormal sound.
Aiming at solving the above-described problem, there has been proposed a chain tensioner configured to control the amount of setback of the plunger.
For example, Patent Document 1 discloses a chain tensioner configured to limit the setback of the plunger while making use of a ratchet.
Patent Document 2 discloses a chain tensioner configured to limit the setback of the plunger, by allowing a resister clip, for clamping a circumferential groove formed on the plunger, to come into contact with the rear side face of a guide groove formed on the cylinder portion of the housing, and by further allowing the engagement surface of the circumferential groove, provided to the outer circumference of the plunger, to come into contact with the resister clip.
Patent Document 3 discloses a chain tensioner configured to have a lock mechanism, which inhibits setback of the plunger making use of a piston engageable, in the orthogonal direction, with a rack formed on the circumferential surface of the plunger, and which can be unlocked by applying hydraulic pressure to the piston.
Patent Document 4 discloses a chain tensioner having a tension rod and a flat washer having a through-hole allowing therethrough insertion of the tension rod, and configured to enable and disable movement of the tension rod, by varying inclination of the flat washer under frictional force applied thereto, by forward and setbacks of the tension rod.
The chain tensioner disclosed in Patent Document 1, however, needs be provided with a large ratchet having an intricate structure. The chain tensioner disclosed in Patent Document 2 needs formation of a plurality of grooves having complex geometries. The chain tensioner disclosed in Patent Document 3 needs formation of the rack, so that the locking mechanism may be more complicated, and may need a larger number of components. The chain tensioner disclosed in Patent Document 4 is configured to disable motion of the tension rod, by applying frictional force to the flat washer by forward and setbacks, to thereby vary the inclination of the flat washer. For this reason, a strict dimensional accuracy may be required for the outer diameter of the tension rod and the inner diameter of the flat washer, in order to ensure normal operation. In addition, a stopper pin may be necessary in order to allow forward motion of the tension rod from the initial state, or the idle state, so that the housing needs be machined to accommodate the stopper pin. As has been described in the above, all prior arts suffer from problems in that the structure and processes may be complicated, and thereby the cost for manufacturing may increase.
The present invention was conceived after considering the above-described situation, an object of which is to provide a chain tensioner configured to limit the amount of setback of the plunger, which is simple in the structure, readily manufacturable, and capable of reducing the cost.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-263779    Patent Document 2: Japanese Patent Application Laid-Open No. 2001-146946    Patent Document 3: Japanese Utility Application Laid-Open No. 5-86057    Patent Document 4: Japanese Patent Application Laid-Open No. 2-309042